1. Field of the invention
The present invention relates to a hydraulic active suspension system for a vehicle such as an automobile or the like, and, more particularly, relates to a hydraulic active suspension system which controls both vehicle height and the attitude of a vehicle body.
2. Prior art
As one of the active suspension systems for a vehicle such as an automobile or the like, a hydraulic active suspension system has already been known which comprises a plurality of hydraulic actuators each being provided corresponding to vehicle wheels and adapted to increase and decrease the vehicle height as respective results of the supply and the discharge of working fluid to and from its working fluid chamber, means for supplying working fluid to and discharging it from the working fluid chambers, and control means for controlling the supplying and discharging means in accordance with the running conditions of the vehicle.
As described, for example, in Japanese Patent Laid Open Publication No. 60-500662 (1985), in such a hydraulic active suspension system, it has already been known in the art to control the supplying and discharging means so that the pressure within the working fluid chamber in each actuator may be a value corresponding to the sum of a basic pressure which is determined according to the running conditions of a vehicle and a compensating pressure which is determined based upon the integral value of the difference between a standard vehicle height and an actual vehicle height. According to the hydraulic active suspension system of this type, vehicle height adjustment can be performed in a more stable manner as compared with the system in which the pressure within the working fluid chamber in each actuator is not controlled with a compensating pressure determined according to the integral value of the vehicle height difference.
In such a hydraulic active suspension system as mentioned just above, however, under the condition where the difference between an actual vehicle height and a standard vehicle height is very high due to an extremely high loaded weight, the integral value of the vehicle height difference will inevitably be very high, resulting in too much high compensating value in pressure being determined according to the integral value. Therefore, the control on the pressure within the working fluid chamber in each actuator based upon the compensating value in pressure determined according to the integral value of the vehicle height difference would override the pressure control based upon the basic pressure determined according to the running conditions of the vehicle.